1. Technical Field
The present invention relates to a multicarrier signal, a method of tracking a transmission channel on the basis of such a signal and a device therefor.
It concerns the field of digital transmissions (digital radio transmissions), and finds applications in particular in receivers of systems for digital radiocommunications with mobiles, for example professional radiocommunications systems (PMR systems, the abbreviation standing for “Professional Mobile Radio”).
2. Related Art
In these systems, the digital data are transmitted by modulation of a radiofrequency carrier wave. Stated otherwise, a radio signal is sent over the transmission channel, this signal being modulated so as to carry the digital information to be transmitted.
One seeks to implement modulation techniques that offer better resistance with regard to disturbances undergone by the radio signal during its transmission through the transmission channel. In essence, these disturbances originate:                on the one hand from the fading phenomenon, which is frequency selective as soon as the coherence band is overstepped (one speaks in this first case of selective fading), but which is not frequency selective once the width of the channel is less than the coherence band (one speaks in this latter case of flat fading). This fading phenomenon is due to the propagation multipaths which give rise to intersymbol interference (ISI) also known as intersymbol distortion. Selective fading is the image in the frequency domain of propagation multipaths exhibiting large delays between paths (the maximum delay between the paths is such that the inverse of this delay is less than the width of the band of the signal). Flat fading occurs when the delay between the propagation multipaths is small and when the maximum delay between the paths is such that the inverse of this delay is greater than the width of the band of the signal.        on the other hand, the amplitude and the phase of the or of each of the propagation paths may be static (in the sense that they do not vary in the course of time) or on the contrary dynamic (when the propagation conditions vary in the course of time). In the dynamic case, the frequency of this phenomenon (also called the frequency of the fading) and, more generally, the frequency spectrum of the fading are related to the speed of the mobile and to the carrier frequency of the signal sent. The conventional model adopted for the power spectrum of the fading is described in the work “Microwave Mobile Communications”, by William C. Jakes, Jr., published by John Wiley & Sons, 1974, pp. 19-25), and involves the Doppler frequency fD given by:        
      f    D    =            V      c        ×          f      c      where V is the speed of the mobile, c is the speed of light, and fc the frequency of the carrier.
The power spectral density of the fading is therefore:
            p      fading        ⁡          (      f      )        =            p      π        ×          1              f        D              ×          1                        (                      1            -                                          (                                  f                  /                                      f                    D                                                  )                            2                                )                    with Pfading(f) the power of the fading, that is to say the power of the signal received.
There is currently effort to seek to implement a multicarrier modulation called OFDM (standing for “Orthogonal Frequency Multiplexing”). This modulation technique has been adopted for the European standard regarding digital audio broadcasting systems (DAB systems, the abbreviation standing for “Digital Audio Broadcasting”). It consists in distributing the data to be transmitted over a set of subcarriers sent in parallel in the radio signal. This results in a flat fading effect in relation to each subcarrier since the bandwidth of each subcarrier is less than the coherence band. Furthermore, it results in a reduction in the sensitivity of transmission in relation to the phenomenon of multipaths.
Nevertheless, the OFDM technique has certain constraints in the applications of the type of those envisaged above, in which the spectral efficiency of the transmission is a key characteristic.
Specifically, the signal to be transmitted is constructed on a time-frequency lattice. The signal is structured framewise, frames being transmitted successively through the transmission channel. Each frame comprises a number M of adjacent subcarriers within a channel of specific spectral width, each of these subcarriers being divided into N time intervals, called symbol times. The duration of a symbol time corresponds to the duration of transmission of a symbol. A frame of the signal therefore comprises M×N symbols. It is recalled that a symbol corresponds to a specific number of information bits, for example eight bits, which takes a specific value in an ad hoc alphabet.
However, it is necessary to introduce pilot symbols into the frame so as to allow the tracking of the transmission channel by the receiver. It is recalled that a pilot symbol is a symbol introduced into the frame by the sender, at a location and with a value which are known to the receiver. Channel tracking is carried out by a sequence of steps implemented by the receiver before the actual demodulation of the signal received. This sequence of steps comprises, on the one hand, the estimation of the value of the channel (that is to say the value of the amplitude and of the phase of the channel) for the pilot symbols of the frame, which produces values of the channel that are estimated for all the pilot symbols of the frame. It further comprises an interpolation of the value of the channel for the other symbols of the frame, which produces M×N values of the channel that are interpolated for all the symbols of the frame. According to a customary interpolation process, the values of the channel that are interpolated for the pilot symbols of the frame correspond to the values of the channel that are estimated for these symbols (stated otherwise, the interpolation step conserves, for the pilot symbols, the values produced by the estimation step).
The tracking of the channel is aimed at estimating the disturbances undergone by the symbols during the transmission of the frame through the transmission channel, which result in particular from the aforesaid two phenomena. One thus generates a matrix of interpolated values, which is an M×N matrix, which is used for the actual demodulation of the frame received.
The presence of the pilot symbols in the frame generates an overhead, which penalizes the useful throughput of the transmission (generally expressed as a number of symbols per second).
In the state of the art, the tracking of the channel is carried out subcarrier by subcarrier. For this purpose, each subcarrier contains pilot symbols, and a temporal interpolation is performed for each subcarrier on the basis of the values of the channel that are estimated from the pilot symbols that it contains.